Some unidirectional condenser microphone unit that are used for conferences, for example, each include an audio-signal outputting printed circuit board (PCB) attached to the rear portion of a microphone unit (hereinafter, referred to as “unit”) to downsize the condenser microphone. The external housing of the condenser microphone consists of three components; a cap covering the front end (adjacent to a sound source) of the unit, a cylindrical shaped microphone case accommodating the unit and the PCB, and the rear case surrounding the rear portion (remote from the sound source) of the PCB. A microphone cable extracting audio-signals output from the condenser microphone is disposed in the rear case. For a gooseneck microphone, a flexible pipe is connected with the rear case.
The three components of the external housing of the condenser microphone are made of metal and are coupled so that the external housing forms an electrostatic shield. If the electrical connections among the coupling portions are unstable, intense electromagnetic waves applied to such coupling portions generate radio frequency (RF) current, which intrudes into the interior of the condenser microphone. As a result, the unit detects the RF current to output it as noise. The noise output from the unit increases if the electrical connection is unstable, in particular, between the microphone case and the rear case or between the PCB and the rear case.
In the conventional condenser microphone, the microphone case is coupled to the rear case with several screws (three screws, for example) that are screwed from the outer periphery of the microphone case. The microphone case is in contact with the rear case only at parts of the inner periphery deformed by the screwed screws. A larger number of screws used to couple the microphone case to the rear case enhance the electrical connection between the microphone case and the rear case, however, this leads to an inferior appearance of the condenser microphone.
In the conventional condenser microphone, the PCB is coupled to the rear case by which a metal plate shaped spring, for example, attached to the PCB is in contact with the rear case. The microphone is a compact microphone including a microphone case having an outer diameter of approximately 12 mm. Therefore, it is difficult through a mechanical process to form the rear case into a structure for mechanically fixed to the PCB.
FIG. 12 is an exploded perspective view of a conventional condenser microphone. The condenser microphone includes a cap 2, a microphone case 3, a unit 10, a PCB 200, and a rear case 300.
FIG. 13 is an exploded side cross-sectional view of the condenser microphone illustrated in FIG. 12.
The microphone case 3 is made of metal and is in a hollow cylindrical shape. The cap 2 is attached to a front portion of the microphone case 3 so as to cover the open front end of the microphone case 3 from the outside of the microphone case 3. Thread holes 3h are formed on the surface of the rear portion of the microphone case 3. Screws 40 (illustrated in FIG. 12) are inserted into these thread holes 3h. 
The unit 10 includes a unit case that is made of metal and has an opening, a diaphragm and a fixed electrode constitute a capacitor, and a circuit board that converts a variation in electrostatic capacitance generated between the diaphragm and the fixed electrode into electric signals. The unit case accommodates the capacitor and the circuit board. A sound introducing hole through which sound waves from the sound source pass is formed at the bottom surface (a surface opposite to the opening) of the unit case. The circuit board is fixed in the interior of unit case so as to close the opening of the unit case from the inside of the unit case by curling of the rear edge portion of the unit case.
The PCB 200 is in a rectangular plate shape in plan view. The PCB 200 has cutouts 222 at the front end portion in connection with the unit 10.
The rear case 300 is made of metal and includes a large-diameter portion 331, a middle-diameter portion 332, a small-diameter portion 333, and a flange 334. Deep grooves 331a, receiving the rear portion of the PCB 200, and thread holes 331, communicating with the thread holes 3h, are formed on the large-diameter portion 331.
FIG. 14 is a side cross-sectional view of the condenser microphone illustrated in FIG. 12. The rear end of the microphone case 3 abuts on the flange 334 of the rear case 300 such that the microphone case 3 is coupled to the rear case 300. The rear portion of the PCB 200 is received in the deep grooves 331a of the rear case 300. The unit 10 and the PCB 200 are accommodated in the microphone case 3.
A plate spring 400 is made of metal and electrically connects the PCB 200 and the rear case 300. The plate spring 400 is fixed to the PCB 200 and the rear case 300. For example, one end of the plate spring 400 is attached to the rear case 300, while the other end of the plate spring 400 is disposed between the rear portion of the PCB 200 and the deep grooves 331a. 
FIG. 15 is a cross-sectional plan view of the condenser microphone illustrated in FIG. 12. The microphone case 3 is electrically connected with the rear case 300 through the screws 40.
The use of such a metal plate spring to establish stable electrical connection between the PCB and the rear case is disclosed (for example, refer to Japanese Patent No. 4417801).